Measles remains one of the leading causes of children death. Measles virus (MV) interacts with two cellular receptors: the immune cell-specific protein signaling lymphocyte activation molecule (SLAM) and the ubiquitous regulator of complement activation membrane cofactor protein (MCP;CD46). Wild type MV use SLAM as a port of cell entry;the live vaccine strain Edmonston efficiently uses CD46 in addition. Even if strong CD46 interactions correlate with MV attenuation, other pathogens (three viruses and two bacteria) target CD46, suggesting that CD46 facilitates pathogen's spread. The research proposed here will further characterize the mechanisms of engagement of the receptors and assess the consequences of these interactions for infection progression. We will test two hypotheses: first, that binding SLAM with a given affinity sustains rapid progression of infection in selected types of immune cells. Second, that low affinity CD46 interactions favor virus spread, but high affinity interactions interfere with it. Our experimental plan foresees three aims. In aim 1 we will characterize how the viral attachment protein engages its receptors: we will build models of the surfaces of the MV H, SLAM and CD46 proteins that are buried after contact. In parallel we will produce soluble mutant protein ectodomains, measure their binding kinetics by surface plasmon resonance, and use the results to instruct further cycles of model building and testing. To understand the effects of changes in binding kinetics on membrane fusion and cell entry we will assess the competence of mutant membrane-bound H proteins in supporting membrane fusion, and in substituting the standard H protein in viruses. Aim 2 will assess the role of SLAM and CD46 interactions for MV infection of human lymphoid cells. We will infect blocks of tissues from human tonsils that retain an organ-like architecture, and peripheral blood mononuclear cells, with viruses with selective receptor specificity and assess whether they have the predicted cell-specific pattern of replication. Aim 3 will appraise how important CD46 interactions are for the progression of MV infection and epithelia invasion in an organism. We will compare infections of wild type and vaccine MV in mice that do or don't express CD46 in addition to SLAM and assess whether CD46 interactions have a positive or negative effect on virus spread. RELEVANCE OF THIS RESEARCH TO PUBLIC HEALTH Measles still causes the death of about 600,000 children yearly. Receptor engagement, the initial interaction between a virus and its host, is a critical determinant of disease outcome. More detailed understanding of how measles virus engages its receptors will facilitate the design of more attenuated vaccine strains for use in immunosuppressed patients. It will also boost further development of oncolytic viruses, targeted.